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                <p>在讲解内存管理之前，我们首先介绍一下虚拟内存。</p>
<p>在没有虚拟内存之前，如果内存中同时运行的两个程序都在同一个位置上面写入值，那么第二个程序写的值就会覆盖第一个程序写的值，可能之后第一个程序继续在该位置上面写值又会覆盖第二个程序写的值，那这样就会导致两个程序都崩溃。</p>
<p>那怎么解决这种问题呢？于是虚拟内存就诞生了。</p>
<p>我们可以把进程所使用的地址「隔离」开来，即让操作系统为每个进程分配独立的一套「<strong>虚拟地址</strong>」，各个进程之间互不干涉，但是有个前提每个进程都不能直接访问物理地址，至于虚拟地址最终怎么落到物理内存里，对进程来说是透明的，由操作系统管理。</p>
<p>操作系统会提供一种机制，将不同进程的虚拟地址和不同内存的物理地址映射起来。如果程序要访问虚拟地址的时候，由操作系统转换成不同的物理地址，这样不同的进程运行的时候，写入的是不同的物理地址，这样就不会冲突了。</p>
<p>于是，这里就引出了两种地址的概念：</p>
<ul>
<li>我们程序所使用的内存地址叫做<strong>虚拟内存地址</strong>（<em>Virtual Memory Address</em>）</li>
<li>实际存在硬件里面的空间地址叫<strong>物理内存地址</strong>（<em>Physical Memory Address</em>）。</li>
</ul>
<p>操作系统引入了虚拟内存，进程持有的虚拟地址会通过 CPU 芯片中的内存管理单元（MMU）的映射关系，来转换变成物理地址，然后再通过物理地址访问内存，如下图所示：</p>
<img src="https://cdn.jsdelivr.net/gh/kuangtianyu/img/img/image-20211216204946974.png" alt="虚拟地址寻址" style="zoom:80%;" />

<p>那么操作系统是如何管理虚拟地址与物理地址之间的关系？</p>
<p>主要有两种方式，分别是<strong>内存分段和内存分页</strong>，分段是比较早提出的，我们先来看看内存分段。</p>
<h2 id="内存分段"><a href="#内存分段" class="headerlink" title="内存分段"></a>内存分段</h2><p>程序是由若干个逻辑分段组成的，如可由代码分段、数据分段、栈段、堆段组成。<strong>不同的段是有不同的属性的，所以就用分段（Segmentation）的形式把这些段分离出来。</strong></p>
<p>那么分段机制下，虚拟地址和物理地址是如何映射的？</p>
<p>分段机制下的虚拟地址由两部分组成，<strong>段选择子</strong>和<strong>段内偏移量</strong>，具体如下图所示：</p>
<img src="https://cdn.jsdelivr.net/gh/kuangtianyu/img/img/image-20211216210516761.png" alt="内存分段-寻址的方式" style="zoom:80%;" />

<ul>
<li><strong>段选择子</strong>就保存在段寄存器里面。段选择子里面最重要的是<strong>段号</strong>，用作段表的索引。<strong>段表</strong>里面保存的是这个<strong>段的基地址、段的界限和特权等级</strong>等。</li>
<li>虚拟地址中的<strong>段内偏移量</strong>应该位于 0 和段界限之间，如果段内偏移量是合法的，就将段基地址加上段内偏移量得到物理内存地址。</li>
</ul>
<p>在上面了，知道了虚拟地址是通过<strong>段表</strong>与物理地址进行映射的，分段机制会把程序的虚拟地址分成 4 个段，每个段在段表中有一个项，在这一项找到段的基地址，再加上偏移量，于是就能找到物理内存中的地址，如下图：</p>
<img src="https://cdn.jsdelivr.net/gh/kuangtianyu/img/img/image-20211216211211762.png" alt="内存分段-虚拟地址与物理地址" style="zoom:80%;" />

<p>如果要访问段 3 中偏移量 500 的虚拟地址，我们可以计算出物理地址为，段 3 基地址 7000 + 偏移量 500 = 7500。</p>
<p>分段的办法很好，解决了程序本身不需要关心具体的物理内存地址的问题，但它也有一些不足之处：</p>
<ul>
<li>第一个就是<strong>内存碎片</strong>的问题。</li>
<li>第二个就是<strong>内存交换的效率低</strong>的问题。</li>
</ul>
<p>首先看看分段为什么会产生内存碎片问题，假设有 1G 的物理内存，用户执行了多个程序，其中：</p>
<ul>
<li>游戏占用了 512MB 内存</li>
<li>浏览器占用了 128MB 内存</li>
<li>音乐占用了 256 MB 内存</li>
</ul>
<p>这个时候，如果我们关闭了浏览器，则空闲内存还有 1024 - 512 - 256 = 256MB。</p>
<p>如果这个 256MB 不是连续的，被分成了两段 128 MB 内存，这就会导致没有空间再打开一个 200MB 的程序，即使我们的空闲内存是大于 200 MB 的，就像下图这样：</p>
<img src="https://cdn.jsdelivr.net/gh/kuangtianyu/img/img/image-20211216211531608.png" alt="内存碎片问题" style="zoom:80%;" />

<p>那解决的方式就是内存交换。</p>
<p>可以把音乐程序占用的那 256MB 内存写到硬盘上，然后再从硬盘上读回来到内存里。不过再读回的时候，我们不能装载回原来的位置，而是紧紧跟着那已经被占用了的 512MB 内存后面。这样就能空缺出连续的 256MB 空间，于是新的 200MB 程序就可以装载进来。</p>
<p>接下来再看看效率低的问题：</p>
<p>对于多进程的系统来说，用分段的方式，内存碎片是很容易产生的，产生了内存碎片，那不得不重新 <code>Swap</code> 内存区域，这个过程会产生性能瓶颈。</p>
<p>因为硬盘的访问速度要比内存慢太多了，每一次内存交换，我们都需要把一大段连续的内存数据写到硬盘上。所以，<strong>如果内存交换的时候，交换的是一个占内存空间很大的程序，这样整个机器都会显得卡顿。</strong>为了解决内存分段的内存碎片和内存交换效率低的问题，就出现了内存分页。</p>
<h2 id="内存分页"><a href="#内存分页" class="headerlink" title="内存分页"></a>内存分页</h2><p>分段的好处就是能产生连续的内存空间，但是会出现内存碎片和内存交换的空间太大的问题。</p>
<p>要解决这些问题，那么就要想出能少出现一些内存碎片的办法。另外，当需要进行内存交换的时候，让需要交换写入或者从磁盘装载的数据更少一点，这样就可以解决问题了。这个办法，也就是<strong>内存分页</strong>（<em>Paging</em>）。</p>
<p><strong>分页是把整个虚拟和物理内存空间切成一段段固定尺寸的大小</strong>。这样一个连续并且尺寸固定的内存空间，我们叫<strong>页</strong>（<em>Page</em>）。在 Linux 下，每一页的大小为 <code>4KB</code>。</p>
<p>虚拟地址与物理地址之间通过<strong>页表</strong>来映射，如下图：</p>
<img src="https://cdn.jsdelivr.net/gh/kuangtianyu/img/img/image-20211216211929247.png" alt="内存映射" style="zoom:80%;" />

<p>页表实际上存储在 CPU 的<strong>内存管理单元</strong> （<em>MMU</em>） 中，于是 CPU 就可以直接通过 MMU，找出要实际要访问的物理内存地址。</p>
<p>而当进程访问的虚拟地址在页表中查不到时，系统会产生一个<strong>缺页异常</strong>，进入系统内核空间分配物理内存、更新进程页表，最后再返回用户空间，恢复进程的运行。</p>
<p>那么分页是怎么解决分段的内存碎片、内存交换效率低的问题？</p>
<p>由于内存空间都是预先划分好的，也就不会像分段会产生间隙非常小的内存，这正是分段会产生内存碎片的原因。而<strong>采用了分页，那么释放的内存都是以页为单位释放的，也就不会产生无法给进程使用的小内存。</strong></p>
<p>如果内存空间不够，操作系统会把其他正在运行的进程中的「最近没被使用」的内存页面给释放掉，也就是暂时写在硬盘上，称为<strong>换出</strong>（<em>Swap Out</em>）。一旦需要的时候，再加载进来，称为<strong>换入</strong>（<em>Swap In</em>）。所以，一次性写入磁盘的也只有少数的一个页或者几个页，不会花太多时间，<strong>内存交换的效率就相对比较高。</strong></p>
<img src="https://cdn.jsdelivr.net/gh/kuangtianyu/img/img/image-20211216214541602.png" alt="换入换出" style="zoom:80%;" />

<p>更进一步地，分页的方式使得我们在加载程序的时候，不再需要一次性都把程序加载到物理内存中。我们完全可以在进行虚拟内存和物理内存的页之间的映射之后，并不真的把页加载到物理内存里，而是<strong>只有在程序运行中，需要用到对应虚拟内存页里面的指令和数据时，再加载到物理内存里面去。</strong></p>
<p>那么分页机制下，虚拟地址和物理地址是如何映射的呢？</p>
<p>在分页机制下，虚拟地址分为两部分，<strong>页号</strong>和<strong>页内偏移</strong>。页号作为页表的索引，<strong>页表</strong>包含物理页每页所在<strong>物理内存的基地址</strong>，这个基地址与页内偏移的组合就形成了物理内存地址，见下图。</p>
<img src="https://cdn.jsdelivr.net/gh/kuangtianyu/img/img/image-20211216214851260.png" alt="内存分页寻址" style="zoom:80%;" />

<p>总结一下，对于一个内存地址转换，其实就是这样三个步骤：</p>
<ul>
<li>把虚拟内存地址，切分成页号和偏移量；</li>
<li>根据页号，从页表里面，查询对应的物理页号；</li>
<li>直接拿物理页号，加上前面的偏移量，就得到了物理内存地址。</li>
</ul>
<p>下面举个例子，虚拟内存中的页通过页表映射为了物理内存中的页，如下图：</p>
<img src="https://cdn.jsdelivr.net/gh/kuangtianyu/img/img/image-20211216215114034.png" alt="虚拟页与物理页的映射" style="zoom:80%;" />

<p>这看起来似乎没什么毛病，但是放到实际中操作系统，这种简单的分页是肯定是会有问题的。那么简单的分页有什么缺陷呢？</p>
<p>有空间上的缺陷，因为操作系统是可以同时运行非常多的进程的，那这不就意味着页表会非常的庞大。在 32 位的环境下，虚拟地址空间共有 4GB，假设一个页的大小是 4KB（2^12），那么就需要大约 100 万 （2^20） 个页，每个「页表项」需要 4 个字节大小来存储，那么整个 4GB 空间的映射就需要有 <code>4MB</code> 的内存来存储页表。</p>
<p>这 4MB 大小的页表，看起来也不是很大。但是要知道每个进程都是有自己的虚拟地址空间的，也就说都有自己的页表。那么，<code>100</code> 个进程的话，就需要 <code>400MB</code> 的内存来存储页表，这是非常大的内存了，更别说 64 位的环境了。</p>
<p>我们可以使用多级页表来解决。</p>
<p>对于单页表的实现方式，在 32 位和页大小 <code>4KB</code> 的环境下，一个进程的页表需要装下 100 多万个「页表项」，并且每个页表项是占用 4 字节大小的，于是相当于每个页表需占用 4MB 大小的空间。</p>
<p>我们把这个 100 多万个「页表项」的单级页表再分页，将页表（一级页表）分为 <code>1024</code> 个页表（二级页表），每个表（二级页表）中包含 <code>1024</code> 个「页表项」，形成<strong>二级分页</strong>。如下图所示：</p>
<img src="https://cdn.jsdelivr.net/gh/kuangtianyu/img/img/image-20211216215954278.png" alt="二级分页" style="zoom:80%;" />

<p>操作系统只会在需要二级页表的时候才会创建二级页表。</p>
<p>这里我们简单的看一下 TLB：</p>
<p>多级页表虽然解决了空间上的问题，但是虚拟地址到物理地址的转换就多了几道转换的工序，这显然就降低了这俩地址转换的速度，也就是带来了时间上的开销。</p>
<p>程序是有局部性的，即在一段时间内，整个程序的执行仅限于程序中的某一部分。相应地，执行所访问的存储空间也局限于某个内存区域。</p>
<p>我们就可以利用这一特性，把最常访问的几个页表项存储到访问速度更快的硬件，于是计算机科学家们，就在 CPU 芯片中，加入了一个专门存放程序最常访问的页表项的 Cache，这个 Cache 就是 TLB（<em>Translation Lookaside Buffer</em>） ，通常称为页表缓存、转址旁路缓存、快表等。就像下图这样：</p>
<img src="https://cdn.jsdelivr.net/gh/kuangtianyu/img/img/image-20211221091817726.png" alt="地址转换" style="zoom:80%;" />

<p>在 CPU 芯片里面，封装了内存管理单元（<em>Memory Management Unit</em>）芯片，它用来完成地址转换和 TLB 的访问与交互。有了 TLB 后，那么 CPU 在寻址时，会先查 TLB，如果没找到，才会继续查常规的页表。</p>
<p>TLB 的命中率其实是很高的，因为程序最常访问的页就那么几个。</p>
<h2 id="段页式内存管理"><a href="#段页式内存管理" class="headerlink" title="段页式内存管理"></a>段页式内存管理</h2><p>内存分段和内存分页并不是对立的，它们是可以组合起来在同一个系统中使用的，那么组合起来后，通常称为<strong>段页式内存管理</strong>。</p>
<p>段页式内存管理实现的方式：</p>
<ul>
<li>先将程序划分为多个有逻辑意义的段，也就是前面提到的分段机制；</li>
<li>接着再把每个段划分为多个页，也就是对分段划分出来的连续空间，再划分固定大小的页；</li>
</ul>
<p>这样，地址结构就由<strong>段号、段内页号和页内位移</strong>三部分组成。</p>
<p>用于段页式地址变换的数据结构是每一个程序一张段表，每个段又建立一张页表，段表中的地址是页表的起始地址，而页表中的地址则为某页的物理页号，如图所示：</p>
<img src="https://cdn.jsdelivr.net/gh/kuangtianyu/img/img/image-20211221094121005.png" alt="段页式管理中的段表、页表与内存的关系" style="zoom:80%;" />

<p>段页式地址变换中要得到物理地址须经过三次内存访问：</p>
<ul>
<li>第一次访问段表，得到页表起始地址；</li>
<li>第二次访问页表，得到物理页号；</li>
<li>第三次将物理页号与页内位移组合，得到物理地址。</li>
</ul>
<p>可用软、硬件相结合的方法实现段页式地址变换，这样虽然增加了硬件成本和系统开销，但提高了内存的利用率。</p>
<p>好啦，到这里我们的虚拟内存就讲解完了，你记住了吗？</p>
<blockquote>
<p>引用：</p>
<p><a target="_blank" rel="noopener" href="https://mp.weixin.qq.com/s?__biz=MzUxODAzNDg4NQ==&amp;mid=2247485033&amp;idx=1&amp;sn=bf9ba7aca126ad186922c57a96928593&amp;chksm=f98e42c3cef9cbd514df38d04deb5e7a9ea67dbd478da75fc4a7636ee90b1384d65f68eb23f5&amp;scene=178&amp;cur_album_id=1408057986861416450#rd">https://mp.weixin.qq.com/s?__biz=MzUxODAzNDg4NQ==&amp;mid=2247485033&amp;idx=1&amp;sn=bf9ba7aca126ad186922c57a96928593&amp;chksm=f98e42c3cef9cbd514df38d04deb5e7a9ea67dbd478da75fc4a7636ee90b1384d65f68eb23f5&amp;scene=178&amp;cur_album_id=1408057986861416450#rd</a></p>
</blockquote>

                
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